1. Field of the Invention
The present invention relates to a coating for providing a cover sheet or face paper used in the manufacture of wallboard with a light colored finishing surface, and a wallboard using the same. More particularly, the present invention relates to a cover sheet composed of at least one ply of generally unbleached recycled paper which utilizes a coating in place of manila face plies to obtain a light colored finish.
2. Description of the Prior Art
Paper for gypsum board is conventionally made by pulping up waste paper constituents of old corrugated paper, or kraft cuttings and waste newsprint. In cleaning, screening and refining the suspended materials in water suspension, the processed paper stock is diluted still further with water and then formed by draining the plies of paper on several continuously moving wire cylinders, where the separate plies are joined together by a carrying felt, or, alternatively, by depositing one or more paper stock slurries on a single flat moving drainage wire of a Fourdrinier-type machine to created a single or multi-ply paper web. The weak paper web is then dewatered (dried) in a press section where water is pressed out of the web. The pressed paper is dried in a multi-cylinder drying section with steam added to each cylinder. The dried paper is subjected to a squeezing or calendering operation for uniformity in thickness and is then finally wound into rolls. Subsequently, this paper is utilized as paper cover sheets to form gypsum wallboard by depositing a calcined gypsum slurry composed of calcium sulphate hemihydrate upon a reverse side of the face paper, then placing the backing paper upon the slurry, and permitting the gypsum to set and dry. A more detailed description of a conventional method of manufacturing wallboard is disclosed in an article by Frank C. Appleyard entitled "Construction Materials", Industrial Minerals and Rocks, 5th Ed, AIME, 1983, and in U.S. Pat. No. 3,307,987, "Process of Making Gypsum Wallboard Having a Decreased Starch Content in the Gypsum Core." These references are now incorporated by reference.
Present day cover sheets which are used in the production of gypsum wallboard are of two kinds. The first is used for the paper cover sheets which are used for the back side of the wallboard, is formed of recycled wastepaper having many contaminants and has a generally dark color. The second type is used on the outer face side of the wallboard. This is the decorated side. It is generally formed as a multi-ply sheet with 2 to 7 filler plies which include clean hard stock waste paper furnishes such as kraft and boxboard cuttings. The color of the filler plies varies, though it has a generally dark appearance.
To provide a light, manila colored, smooth appearance on the exposed or decorated side, 1 to 2 top liner plies are provided, covering the filler plies. The top liner plies (manila face plies) consist mainly of soft stock furnishes such as flyleaf shavings and newspaper. A light colored finish is desirable for the face paper because it may be covered with relatively few coats of paint without the risk of the underlying color "bleeding" through the paint. The flyleaf shavings can be described as baled trim of magazines, catalogs and similar printed and unprinted material. Containing predominantly bleached chemical fibers, this face paper material may also contain as much as 10-40% by weight of clays and pigments from coating overlays.
As stated above, the 2 to 7 filler plies of the face cover sheets are formed of clean hard stocks such as kraft and boxboard cuttings. These particular furnish constituents are of such a nature that they are inherently strong. Equally important, they can develop further strength on refining. As a result of these favorable pulp qualities, the final total sheet strength of the gypsum board face cover sheets is predominantly dependent on the furnish make-up of the cover sheet's 2 to 7 filler plies. The refined hard stock fibers which form the filler plies are further identified with easy dewatering (drying), fast drainage, and easy drying. This translates into faster paper production and less energy requirements for drying. In addition, the resultant sheet filler structures are very open or highly porous. This further translates into easier gypsum board drying and faster board line speeds.
In sharp contrast, the furnish components of the 1 to 2 top liner plies which are used to provide a suitable light colored, smooth face appearance are generally formed of 70-80% by weight of flyleaf furnish, old magazine and trim, and 20% waste newsprint. These top liner plies have significantly different characteristics. The soft stock waste paper constituent is inherently weak in strength qualities and has minimal potential for increased strength development. In fact, it has been found that the top liner plies of flyleaf furnish and ground newsprint provide little if any strength to the finished total sheet. The high proportions of inert clays and fillers contained in the flyleaf shavings stock constituent further subtract from the integrity of the liner plies. Moreover, the soft stocks and clays contained in the manila top liner plies are historically noted for poor dewatering (drying), slow drainage, and difficult drying. This results in slower paper production and increased energy requirement for drying.
Briefly summarized, it can readily be seen that the current cellulosic furnish consisting of waste newspapers and flyleaf shavings that typically is used as top liners of the manila gypsum wallboard paper grade contributes to substantial furnish and operational problems. It provides negligible strength to the finished sheet. In addition, the pulp slurry dewaters (dries) with difficulty, thereby adversely affecting the draining and drying conditions of the total filler/liner sheet. This results in poorer machine operability, lower production rates, and increased energy requirements.
Obviously, one corrective measure to obviate these problem conditions would be to replace the current "waste" stocks with "virgin" stocks. This would allow for predictable liner ply strength and would eliminate the critical clays and fillers associated with the flyleaf overlays. However, the cost of doing so would be prohibitive.
Another corrective measure has been proposed by Long, U.S. Pat. No. 4,959,272 assigned to the United States Gypsum Company. Long teaches a paper cover sheet for gypsum wallboard which does not require the use of top liner (manila) plies. Instead, Long uses a coating composed of a light colored mineral filler and a binder which is applied to an exposed surface of the cover sheet. As noted by LaFarge in Australian Patent No. A-19322/92, the paper cover sheet disclosed by Long suffers from several disadvantages.
First, Long's method of producing the cover sheet is not capable of yielding a coating of sufficient thickness to withstand sanding commonly performed to finish joints between adjacent sheets of wallboard. A second disadvantage associated with Long relates to the method of manufacturing the cover sheet. Long teaches precoating the cover sheet at the paper machine as part of the paper making process. Specifically, Long describes applying the coating at the starch box on the calender stack at the dry end of a paper machine. Third, Long's method is disadvantageous as it results in heterogenous distribution of the coating on the surface of the cover sheet. This uneven distribution results in the cover sheet having a non-uniform appearance and also results in variation in porosity across the surface of the cover sheet.
In contrast, LaFarge discloses a conventional cover sheet having manila face plies, but which further includes a mineral coating. LaFarge's coating is provided to retard the yellowing of the face plies due to exposure to sun. However, cover sheets manufactured according to LaFarge are uneconomical as they suffer from all of the disadvantages of conventional cover sheets (poor drying, more expensive cover sheets), and further include the added expense of the coating.
In view of these and other deficiencies of the prior art, it is a general objective of the present invention to provide an improved coated cover sheet for gypsum wallboard which does not require the use of top liner plies to achieve a light colored finish, and which has a coating sufficiently durable to withstand a typical finish sanding operation.
A further objective of the present invention is to provide an improved, durable coating for a gypsum wallboard cover sheet which is sufficiently durable to withstand a sanding operation, and which is sufficiently porous to allow moisture from the core of the gypsum board to evaporate through the cover sheet.
Another objective of the present invention is to provide an improved gypsum wallboard having a finishing surface suitable for painting or vinyl coating and which is uniformly and sufficiently porous to prevent delamination of the cover sheet from the gypsum core.
Still another objective of the present invention is to provide an improved method for applying a durable coating to a cover sheet which does not require a large capital investment.
Yet another objective of the present invention is to provide a method of applying coating to the cover sheet on the wallboard assembly online without appreciably increasing the length of the line or affecting manufacturing speed.